This paper describes how low-lime fly ash can be used to develop chloride-resistant concrete by improving both its physical resistance to the ingress of chlorides and binding capacity of these ions in the cover zone. This includes optimizing the fly ash content, reducing the water/ binder ratio of the concrete, processing fly ash to improve its particulate characteristics and, finally, using ternary blends with silica fume or metakaolin. This last method is shown to provide the highest degree of chloride resistance. A tentative classification of chloride-bearing environments together with recommendations for the specification of concrete for structures exposed to these environments, is proposed.